# Hanging Ball, Electric Fields

1. Jan 17, 2008

### Goldenwind

1. The problem statement, all variables and given/known data
http://session.masteringphysics.com/problemAsset/1001949/13/knight_Figure_25_69.jpg
An electric field 1.00×10^5 N/C causes the ball in the figure to hang at an angle. What is theta?

2. Relevant equations
E = F/q
F = mg

3. The attempt at a solution
The ball isn't accellerating, so it is in equilibrium. Sum of all forces is 0, so the tension force balences gravity + electric field.

E = F/q
F = Eq
F = 100000*(25*0.000001)
Fx = 2.5

F = mg
F = (0.002)(9.81)
Fy = 0.1962

Ft = Sqrt(Fx^2 + Fy^2)
Ft = 2.5076870697916038024546202698347

tan(theta) = Fx/Fy
theta = arctan(2.5 / 0.1962)
theta = 85.5126 degrees

Where is my mistake?

Last edited: Jan 17, 2008
2. Jan 17, 2008

### Shooting Star

You have not given the unit for the electric field, but I'm assuming it's in SI.

> F = 100000*(25*0.000001)

The charge is in nC...

3. Jan 17, 2008

### Goldenwind

Ah. Yeah, when I copied it over from the online problem, it didn't go.
E = 1x10^5 N/C
So I figured I'd convert q into the unit C, therefore get F in the unit N.

4. Jan 17, 2008

### Shooting Star

Did you get the right answer?